The present invention relates to a novel lithium secondary battery having a high input/output performance and suitable for hybrid electric vehicles and the like.
Hybrid electric vehicles, in which an engine and a motor serve as power sources, have been developed and commercialized for environmental protection and energy saving. Moreover, fuel cell hybrid electric vehicles in which a fuel cell is used in place of an engine have been extensively under development for vehicles in the future.
Secondary batteries, which can undergo a number of charging/discharging cycles, are essential devices as power sources for hybrid electric vehicles.
Of secondary batteries, lithium secondary batteries are promising, because of their high operational voltage and capability of generating a high output, and are of increasing importance as power sources for hybrid electric vehicles.
An electrolyte solution for lithium secondary batteries is required to have high voltage-resistant characteristics, and an organic electrolyte solution containing an organic solvent is used.
However, an organic solvent involves disadvantages of poor solubility for lithium salts and large dependence of its conductivity on temperature, thus resulting in a marked deterioration in operational characteristics at low temperatures over those at room temperature.
Carbonate ester compounds are now mainly used as solvents for organic electrolyte solutions for lithium secondary batteries, because of their high voltage-resistant characteristics.
Cyclic carbonate ester solvents have a high viscosity, although they have a high solubility for lithium salts. Linear carbonate ester solvents, on the other hand, have a low solubility for lithium salts, although they have a low viscosity.
Hence, a mixture of cyclic and linear carbonate esters is generally used as the electrolyte solutions.
For improvement of low-temperature characteristics of electrolyte solutions, Patent Documents 1 to 5 shown below disclose use of a linear carbonate ester incorporated with ethyl methyl carbonate (EMC) of an asymmetric structure.
Patent Document 6 shown below discloses use of an acetate ester, which has a lower molecular weight than EMC and works as a solvent of a low viscosity and a low melting point.    Patent Document 1: JP-B-2,705,529    Patent Document 2: JP-A-10-027625    Patent Document 3: JP-A-2001-148258    Patent Document 4: JP-A-2002-305035    Patent Document 5: JP-A-2003-323915    Patent Document 6: JP-A-09-245838
It is an object of the present invention to provide a lithium secondary battery which uses an electrolyte solution exhibiting no inflammability at room temperature (20° C.) and not deteriorating output characteristics of the battery at low temperatures and room temperature.